The present invention relates generally to a combustion control apparatus for an internal combustion engine, and, more particularly, to a combustion control apparatus that switches the combustion mode of an internal combustion engine between the homogeneous-charge combustion mode and the stratified-charge combustion mode in accordance with the running state of the engine.
In an in-cylinder injection type internal combustion engine that injects fuel directly into a cylinder, fuel is injected in the compression stroke immediately before ignition and a flammable air-fuel mixture unevenly distributed only in the vicinity of each spark plug is ignited. This makes it possible to carry out combustion with a lean air-fuel ratio as an average in the whole combustion chamber, i.e., stratified charge combustion. During this stratified charge combustion, the pumping loss can be reduced by narrowing the throttle angle even when the engine is running under a light load where the amount of fuel injection is small. As combustion mainly takes places near each spark plug, the cooling loss can also be reduced, thus resulting in a significant improvement in the fuel efficiency.
In stratified charge combustion, while the overall air-fuel ratio is low, the air-fuel ratio in the vicinity of the spark plug is high so that the combustion temperature near the spark plug is high. Therefore, the amount of NOx produced tends to increase in stratified-charge combustion mode. What is more, because the amount of intake air is large in stratified-charge combustion mode, that tendency grows further. To restrain the production of NOx in stratified charge combustion, therefore, a large amount of EGR (Exhaust Gas Recirculation) gas is supplied.
In the in-cylinder injection type internal combustion engine, fuel is injected in the suction stroke to evenly blend fuel and air in the combustion chamber. This combustion mode is called xe2x80x9chomogeneous charge combustionxe2x80x9d.
Recently, an internal combustion engine which uses an in-cylinder injection system and switches the combustion mode between the homogeneous-charge combustion mode and the stratified-charge combustion mode in accordance with the running state of the engine has been put to a practical use.
At the time of changing the combustion mode, the throttle position or angle, the fuel injection timing, the ignition timing and the angle of an EGR valve provided in the EGR passage where the EGR gas flows are changed. When the combustion mode is switched to the homogeneous-charge combustion mode from the stratified-charge combustion mode, for example, the angle of the EGR valve is controlled to decrease the amount of EGR gas supplied.
It is to be noted however that even after the angle of the EGR valve is changed at the time of mode switching to the homogeneous-charge combustion mode, a large amount of EGR gas remains in the intake manifold or combustion chamber over a certain period of time. Immediately after the stratified-charge combustion mode is switched to the homogeneous-charge combustion mode, therefore, excess EGR gas lowers the combustion speed, thus increasing the possibility of misfire. Misfire causes a torque change, which degrades the drivability.
Japanese Unexamined Patent Publication (KOKAI) No. Hei 8-189405 and Japanese Unexamined Patent Publication (KOKAI) No. Hei 9-195839 disclose a conventional in-cylinder injection type internal combustion engine. At the time the stratified-charge combustion mode is switched to the homogeneous-charge combustion mode in this engine, first, the angle of the EGR valve is changed to the angle that matches with the homogeneous-charge combustion mode. Then, the combustion mode is switched after passing of a delay time sufficient for the EGR gas in the combustion chamber to decrease to the proper amount.
The time the EGR gas remains (remaining time) since the alteration of the angle of the EGR valve varies significantly depending on the running state of the engine at that time. In this respect, the delay time is set long to cope with various running states of the engine. In this case, stratified charge combustion with an insufficient amount of EGR gas continues during the period from the purging of the remaining EGR gas to the switching of the combustion mode. The insufficient EGR gas increases an NOx emission and the combustion speed, which leads to louder stratified-charge combustion noise.
To prevent the NOx emission from increasing and suppress the stratified-charge combustion noise, therefore, it is desirable to switch the combustion mode as early as possible after the angle of the EGR valve is changed. When the delay time is set short, however, depending on the running state of the engine, misfire-originated degradation of the drivability cannot be suppressed sufficiently.
Accordingly, it is an object of the present invention to provide a combustion control apparatus for an internal combustion engine that switches the stratified-charge combustion mode to the homogeneous-charge combustion mode at the proper timing.
To achieve the above object, the present invention provides a combustion control apparatus for an internal combustion engine that has an exhaust manifold, an intake manifold, an EGR passage for connecting the exhaust manifold to the intake manifold, and an EGR valve disposed in the EGR passage, and runs in a stratified-charge combustion mode or a homogeneous-charge combustion mode in accordance with a running state. The combustion control apparatus includes angle changing means, mode switching means, and delay-time setting means. The angle changing means changes an angle of the EGR valve to an angle needed for the homogeneous-charge combustion mode at a time of switching a combustion mode from the stratified-charge combustion mode to the homogeneous-charge combustion mode. The mode switching means switches from the stratified-charge combustion mode to the homogeneous-charge combustion mode when a predetermined delay time elapses after a request for switching the combustion mode has been made. The delay-time setting means sets the delay time based on engine speed and engine load.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.